The Drupal Calendar module is great. You can show view results in a decent-looking calendar easily. However, if you want to show more than what’s provided out of the box, you need to do a bit of undocumented and confusing hacking.

Let’s say that instead of displaying the items, you want to display the number of items. You need to implement hook_calendar_add_items, which, despite its name, actually overrides the items displayed. Your hook_calendar_add_items($view) should return an array of an array of items, which is also confusing, because the receiving function (template_preprocess_calendar in calendar/theme/theme.inc) doesn’t actually do anything with the outermost array, or indeed with multiple entries – it simply takes the last array and sets the items to it.

Your hook_calendar_add_items gets the $view, but not the existing items, so you’ll have to recalculate things using $view->result. Remember: if you return anything, your results will replace the items instead of being added to them.

Because hook_calendar_add_items doesn’t actually do what it advertises, there are probably bugs in this area of code, and this behaviour might change in future releases of Calendar. Be careful when using it. However, it seems to be the easiest way to change Calendar behavior. Caveat developer.